Innovative calcium-doped layered yttrium hydroxides for rapid and efficient removal of fluoride from aqueous solutions: Insight into adsorption mechanism

Fluoride contamination is an important problem for sustainable human development and has to be addressed by environmental governance. However, the rapid and efficient removal of fluoride from aqueous solutions has always been challenging. In this study, several calcium-doped layered yttrium hydroxides (Ca-Y LRHs) were synthesized and used to remove fluoride. Relative to other adsorbents, 6% Ca-Y LRHs exhibits excellent performance of adsorption equilibrium time and maximum monolayer adsorption capacity with 4 min and 125.54 mg/g, respectively, determined by the adsorption kinetics and Langmuir model. The remarkable removal ability is probably attributed to the high fraction of nitrate anions in the interlayer space, which instead of hydroxyl groups or OH-, can be replaced by fluoride, inhibiting the rise of pH in solution. The ions adsorption and detection analysis indicated electrostatic and ion exchange interactions are involved in the defluorination process, and oxygenated groups in interlayer spaces were the main ligands to be replaced by fluoride. Studies on potential applications were successfully conducted on wastewater of an electrochemical fluorination process and surface water, demonstrating the ability of 6% Ca-Y LRHs to remove fluoride from these environments. These findings could provide an alternative strategy for the rapid and efficient treatment of fluoride contamination.